The production of high quality p-n junctions, i.e., with a minimum of interface dislocations, is a major goal in modern semiconductor technology.
Chemical vapor has commonly been used for junction formation, but suffers from the fact that the substrate surface is not sufficiently cleaned and "built-down" (etched), this being basic to proper regrowth of the desired faultless crystal here. This is especially important where an insulator is used as the deposition substrate.
In "III-V" and "II-VI" compounds, forming a junction from a certain lattice match can be established when compounds with related crystal structure are combined, e.g.
InSb and GaSb. PA1 In.sub.x Ga.sub.1.sub.-x Sb PA1 GaAs and AlAs appear very attractive. Here the lattice constants are: PA1 GaAs (N) -- Ga.sub.x Al.sub.1.sub.-x As(P) PA1 GaAs (P) -- Ga.sub.x Al.sub.1.sub.-x As(N)
The difference in lattice constants of these compounds: EQU InSb: a'.sub.o = 6.485 A EQU GaSb: a.sub.o = 6.095 A
is 0.390 A.
But if the interface is formed on account of a variable stoichiometry of the mixed crystal:
The value of x will define the actual lattice mismatch.
[See: H. F. Matare, "Heteroepitaxy," Scientia Electrica, Vol. XV, Fasc. 3 (1969), pp. 95-109.]
The number of dislocations formed at the interface is proportional to the lattice mismatch .delta. = (a'.sub.o - a.sub.o)/a.sub.0, .delta. being a function of x.
In the search for useful material combinations for heterojunctions, the materials:
GaAs: EQU a.sub.o = 5.646 A EQU a'.sub.o = 5.639 A
with a difference a.sub.o - a'.sub.o = 7 .times. 10.sup.-.sup.3 A for a .delta. value of 0.124%.
While the case InSb/GaSb has a basic mismatch of 6.4% and induces a dislocation density of more than 10.sup.10 cm.sup.-.sup.2, the combination GaAs/AlAs includes a dislocation density of only 10.sup.8 cm.sup.-.sup.2. These figures decrease obviously for graded interfaces and, for Ga.sub.x Al.sub.1.sub.-x As, a normal value of x = 0.7 will result in less than 10.sup.5 dislocations cm.sup.-.sup.2 when conditions of growth are optimized.
This optimization is brought about in liquid heteroepitaxy by a graded admixture (i.e., x is graded) in the interface after melt-back -- a "graded junction" resulting. The GaAs of the wafer should be replaced gradually by the ternary compound with decreased x (or increased aluminum content -- i.e., graded doping) exhibited across the growing crystal, this resulting from gradual lowering of the temperature and an accompanying drop in saturation. However, forming such a structure is not feasible where any abrupt temperature change may result (e.g. as in the arrangement described in U.S. Pat. No. 3,628,998). In fact, the present state of the art is not equipped to render such "graded junctions" except by means which are very complex and expensive, using apparatus that is impractical for economic mass production. The present invention is adapted to meet this problem, and to supply such graded junctions in a novel, simplified manner.
Rendering a highly perfect hetero-interface, with proper (optimized) doping, is critically important in the production of modern semi-conductor light emitters. Numerous methods have been designed to fulfill these requirements because the faultless merger of semi-conductors with different band-gaps is at the root of efficient light emitters (see, e.g. H. Kroemer, Proceedings IEEE, Vol. 51, No. 12, pp. 1782-1783, December 1963).
Heretofore, in the forming of related junctions, a perfect monocrystal of a basic compound (e.g. a GaAs wafer) is employed as a starting substrate and is disposed adjacent an appropriate (e.g. Gallium-rich) melt and heat treated such that a portion of another crystal is dissolved, and transported in rather massive fashion, across an intermediate transition layer. Then, upon cooling, one or several layers of related compound crystals are deposited. FIG. 9 could be useful for considering prior art devices comprising a semi-conductive medium 9-M, such as pure gallium Ga, sandwiched between a dopant crystal 9-S (e. g. of gallium arsenide) on one side and a substrate wafer 9-W (for instance of gallium arsenide) on the opposite side thereof so that with a prescribed temperature differential (dT) applied thereacross, with the high temperature at wafer 9-S, a migration of gallium arsenide material will be induced across the transport medium 9-M to build up a deposition layer 9-D on the upper surface of wafer 9-W -- the layer 9-D being deposited on this surface with no melt-back or regrowth as with the invention, of course. Here, as mentioned above, any oxide layer or other contaminant material or surface imperfections on the deposition surface of 9-W will typically interfere with a good, perfect deposition thereon and compromise the characteristics of the resulting junction; for instance, the oxide can produce all too many crystal dislocation sites leading to mismatch of crystals and very poor semi-conductor characteristics, especially for electroluminescence applications.
It is an object of this invention -- rather than simply depositing such crystal layers and rather than using such a transition layer -- to use the substrate wafer as a saturation source, justaposed under a prescribed limited volume melt, then to melt-back to form a correctly composed melt and then to form a graded crystal structure of correct stoichiometry.
A tilting (oscillating) oven (e.g. like one designed by Nelson; see RCA Review 24, 603, 1963) has served as a starting point for dissolution-regrowth work. Such oven manipulation is grossly impractical and ineffective for material transport. This method is the basis for improved multilayer growth methods as described by Panish et al. (M. B. Panish, I. Hayashi, and S. Sumski, Applied Physics Letters, Vol. 16, No. 8, pp. 326-327, 15 Apr. 1970). In the "Panish" method, the surface conditions of the starting wafer are somewhat critical but several layers can be deposited in a continuous process, using a sliding graphite crucible with several melts. This method has its shortcomings, however. A wafer so formed (by the Nelson or Panish techniques) can, on the one hand, readily be left unaffected (untreated); or conversely, the wafer can be taken up and destroyed by the melt -- only a minor shift in conditions can do this- Such techniques, unlike the invention, do not correctly "melt-back", do not use a limited-volume melt, and don't use the substrate wafer as a second source of dopant.
This invention is designed to improve on the foregoing, using a unique stationary, vertical oven with a prescribed temperature gradient and using a limited-volume melt/substrate wafer package, this package being transported through the oven so as to subject it to a very precisely controlled heat cycling -- as a result of which the wafer is melt-back a prescribed distance, corresponding to the melt volume, and recrystallizes to form a junction which is graded in a very precise, desirable manner by a gradual, precisely-controlled cooling -- this being optimized in given instances by "Peltier cooling".
In laser structures with optimized confinement, layer sequences like:
GaAs (N) -- Ga.sub.x Al.sub.1.sub.-x As(N) -- GaAs (N.sup.-) -- Ga.sub.x Al.sub.1.sub.-x As(P) are necessary. (See FIG. 1C)
In recombination radiation diodes, a simple double structure of the form:
can be used to deliver recombination light with a high efficiency because of minimized interface dislocation density and due to the effect of carrier confinement.
Such devices will generally exhibit a band structure as shown in FIG. 1A. Here, a two-junction device 1 is shown (such as may be used as an electro-luminescent device -- e.g. a light emitting diode, LED -- and well known in the art). Device 1 comprises a positive layer 5 (e.g. comprising relative pure GaAs (P) with 10.sup.17 to 10.sup.18 cm.sup.-.sup.3 dopant concentration); and opposite thereto a negative layer 9 (for instance, comprising a mixture of gallium, aluminum and arsenic: Ga.sub.x Al.sub.1.sub.-x As, with x = 0.7 and tellurium as a dopant); plus an intermediate transition layer 7, comprising a transition mixture falling between the mentioned compositions of 5 and 9, to comprise a "graded junction" as will be understood by those skilled in the art. For instance, here, the concentration of gallium will change very gradually from about x = 1.0 at the layer 5/7 interface to about 0.7 at the 7/9 interface; with the aluminum concentration comprising the balance thereof (varying inversely, of course, as known in the art).
Associated curve P-I is a plot of the energy bands (energy levels) in the layers of device 1; the energy bands being generally defined in electron volts.
An electric potential is applied to the opposite faces of device 1 [that is, to a first contact terminal 2' associated with metal plate 3' on the one hand, and to oppositely charged second terminal 2 connected to its contact plate 3, of any suitable shape -- e.g. circular -- with potential being applied thereto to generate radiation from the junction].
Here, the radiation h.nu. will be understood to arise in the interface between layers 5 and 7, proceeding out through layer 9 -- the "high gap side" which has a wider forbidden gap and is transparent to radiation h.nu.. (Layer 7, or transition zone T, is where carrier recombination occurs to generate the radiation h.nu..) Here an initial wafer (GaAs) can represent the "small gap crystal," on which a "wide gap crystal" (Ga.sub.x Al.sub.1.sub.-x As) is deposited so as to form transition zone 7 therebetween. Since a bias in the forward direction here will cause a strong carrier injection, the quasi-Fermi level difference (Ze -- Z.sub.h) places the respective Fermi levels into the bands, and the barrier forms potential wells at the interface (carrier recombination takes place preferentially within the transition zone, causing a high rate of radiative recombination, as necessary for high external quantum efficiency).
The production of these interfaces poses stringent requirements on wetting, melt-back, and regrowth conditions. In a simple dipping procedure, catastrophic problems can arise, some stemming from uncontrolled surface conditions and "melt attack". Similarly, a "solid-liquid-solid" transport growth mode, using only a single source of temperature gradient (like Peltier current), is not sufficiently controllable as to cooling rate. (See U.S. Pat. No. 3,411,946.)
The present invention represents a marked improvement over conventional techniques forming such "graded junctions" with better, more uniform crystal formation and much better yield.